1. Field of the Invention
The present invention relates to a method of fabricating a structure having a beam-like structure that is represented by a comb structure for use in, e.g., a sensor and an actuator manufactured by the MEMS (Micro Electric Mechanical Systems) technique, for example.
2. Description of the Related Art
The term “VC (Vertical Comb) structure” means a structure in which a pair of comb tooth segments of a comb structure are arranged in an interdigitaing relation and one of the comb tooth segments is supported by an elastic member to be displaceable in an out-of-plane direction. Such a structure can be fabricated as a very fine structure with one comb tooth having a width of 5 μm, for example, by employing the micromachining technique called MEMS, which is based on the semiconductor fabrication technique.
The VC structure fabricated by the MEMS is employed in, e.g., a micromirror for conversion of an optical path and a variable capacitor in radio communication equipment. As one practical example, the VC structure can be used as an electrode of an electrostatic comb actuator including an oscillating microstructure which is driven by an electrostatic force.
In the electrostatic comb actuator using the VC structure, a large driving force is required to be generated to displace the comb structure from its initial position. In view of such a requirement, an AVC (Angular Vertical Comb) structure and a method of fabricating the AVC structure are proposed to provide a comb structure capable of increasing the driving force. (See Japanese Patent Laid-Open No. 2004-219839, U.S. Pat. No. 7,085,122, and U.S. Pat. No. 7,089,666.)
An example of the AVC structure will be described below with reference to FIGS. 14A-14C.
In the AVC structure of FIGS. 14A-14C, as viewed from above, a pair of comb tooth segments 301 and 302 (each having teeth) of a comb structure 300 is arranged to interdigitate with each other as shown in FIG. 14A. Also, as viewed from side, one (302) of the comb tooth segments 301 and 302 is arranged inclined so as to be located out of a plane with respect to the other (301) of the comb tooth segments (i.e., out-of-plane arrangement), as shown in FIG. 14B, which is a sectional view taken along a line XIVB-XIVB in FIG. 14A. When a voltage is applied between the comb tooth segments 301 and 302, the inclined comb tooth segment 302 is attracted toward the comb tooth segment 301 by an electrostatic force. Then the inclined comb tooth segment 302 and an oscillating plate 304 are displaced in a direction to rotate about a torsion spring 303, as shown in FIG. 14C, which is a sectional view taken along a line XIVC-XIVC in FIG. 14A.
Known methods of fabricating the AVC structure are as follows:
1. A notch is formed in a comb tooth segment of a comb structure and a resin is filled in the notch. Thereafter, the comb structure is pressed against a die to bend the comb tooth segment. The resin is then cured to fixate the notch in the bent state. (See Japanese Patent Laid-Open No. 2004-219839)
2. After forming a resin portion on a comb tooth segment under an environment at temperature higher than room temperature, the resin portion is cooled to room temperature such that the comb tooth segment is bent by tension generated due to the difference in thermal contraction between the resin and the material of the comb tooth segment. (See U.S. Pat. No. 7,085,122)
3. A comb tooth segment is fabricated by using an elastic material (e.g., single-crystal silicon) that exhibits plasticity at high temperatures. The comb tooth segment is then pressed against a die at a high temperature to plastically deform it. (See U.S. Pat. No. 7,089,666)
The above fabricating method (1) will be briefly described with reference to FIGS. 15A-15C.
An AVC structure 310 according to method (1) is constructed as shown in FIG. 15A. The AVC structure 310 has a pair of comb tooth segments 311 and 316. A support substrate 312 of the comb tooth structure 311 has a notch 313, as shown in FIG. 15B, which is a sectional view taken along a line XVB-XVB in FIG. 15A.
The comb tooth segment 311 is inclined in the following manner. The support substrate 312 is pressed against a die 314 having a slope in order to incline the comb tooth segment 311 by bending the support substrate 312 at the notch 313 together with the comb tooth segment 311, as shown in FIG. 15C, which is a sectional view taken along a line XVC-XVC in FIG. 15A. A reinforcing member 315 made of, e.g., a resin is filled in the notch 313 such that the bent portion is reinforced.
The above fabricating method (2) will be briefly described with reference to FIGS. 16A-16C.
An AVC structure 320 according to the method (2) is constructed as shown in FIG. 16A. The AVC structure 320 has a pair of comb tooth segments 321 and 324. The comb tooth segment 321 is coupled to a support 323 through a hinge 322. The hinge 322 is made of a resin material that is curable under heating.
A manner of inclining the comb tooth segment 321 is illustrated in FIGS. 16B and 16C, which are sectional views respectively taken along lines XVIB-XVIB and XVIC-XVIC in FIG. 16A. First, as shown in FIG. 16B, the hinge 322 made of the above-described resin material is formed between the comb tooth segment 321 and the support 323. Then, the hinge 322 is heated to be cured. The resin undergoes contraction with the progress of curing such that an upper surface of the comb tooth segment 321 is pulled toward the support 323. Based on the contraction curing of the resin material, the comb tooth segment 321 is fixedly held in an inclined state as shown in FIG. 16C.
The above fabricating method (3) will be briefly described with reference to FIGS. 17A-17C.
An AVC structure 330 according to the method (3) is constructed as shown in FIG. 17A. The AVC structure 330 has a pair of comb tooth segments 331 and 336. The comb tooth segment 331 is on one side of an oscillating plate 333 that is supported by a torsion spring 332 and that is able to oscillate in a rotating direction. The torsion spring 332 is made of a material capable of being elastically deformed under heating, such as single-crystal silicon.
A manner of inclining the comb tooth segment 331 is illustrated in FIGS. 17B and 17C, which are sectional views respectively taken along lines XVIIB-XVIIB and XVIIC-XVIIC in FIG. 17A.
As shown in FIG. 17B, a die 335 having a pillar 334 is pressed against the oscillating plate 333 in the direction indicated by a straight arrow and is then heated in a state that the pillar 334 contacts an upper surface of one portion of the oscillating plate 333 on the side opposite to the other portion including the comb tooth segment 331. With the pressing, the oscillating plate 333 is rotated in the direction indicated by a curved arrow, and the torsion spring 332 is plastically deformed into such a shape as holding a certain rotational angle of the oscillating plate 333. Then, the die 335 is removed after cooling. As a result, the oscillating plate 333 is held in the inclined state as shown in FIG. 17C, and the comb tooth segment 331 formed laterally of the oscillating plate 333 is also held in the inclined state together with the oscillating plate 333.
In a structure including a structural segment that is angled relative to a plane parallel to the principal surface of a substrate, it is desired that the angular segment be fabricated so as to incline to match with the designed angle at high accuracy.
With the fabricating method disclosed in Japanese Patent Laid-Open No. 2004-219839 and U.S. Pat. No. 7,085,122, however, control of a bending angle is difficult because the bending angle of the comb tooth segment depends on not only the density and the amount of each of the resins used as materials of the reinforcing member and the hinge, but also the curing temperature and environmental conditions such as an ambient atmosphere. Further, because the thermal expansion coefficient and the amount of moisture absorption of the resin material are larger than those of silicon that is a main material of the comb tooth segment, the bending angle of the comb tooth segment after the fabrication is susceptible to change depending on external environmental conditions such as temperature and humidity.
The fabricating method disclosed in U.S. Pat. No. 7,089,666 has a high probability that, when the pillar is pressed against the oscillating plate, a pressing position is deviated from the designed position. If a deviation occurs, the desired angle is difficult to obtain.